Pneumatic control apparatus



April 3, 1962 A. SILVER 3,027,904

PNEUMATIC CONTROL APPARATUS Original Filed Aug. 25, 1952 2 Sheets-Sheetl ESSOR OUTLET INLET Fig.2. Ffgfi.

, i 24 23 7 PX i & 54

2o l8 l8 ALEXANDER S/L VE'R,

IN VEN TOR.

tates I trite The present invention relates generally to controlapparatus, and is more particularly concerned with pneumatic controlapparatus which relies for its operation upon variations in the ratiobetween predetermined fluid pressures.

This application is a true division of my co-pending application, SerialNo. 306,139, filed August 25, 1952, and entitled Pneumatic ControlApparatus, now Patent No. 2,775,231.

Heretofore, when it was desired to effect large changes in the pressureratio setting of a control mechanism the utilization of controlapparatus of the conventional type has been impractical. -Any solutionof the problem should desirably exclude the necessity for changing theareas of pistons, diaphragms, chambers and the like, or changingleverarea ratios, as such arrangements become overly complicated andimpractical for extremely large pressure ratio changes.

The problem is of particular significance in certain applications, suchas speed governors, and like devices, where a ratio type of control isdesirable whose setting is varied as a function of another variable suchas temperature. The standard bellows and/or diaphragm area ratioarrangement is unsatisfactory where a large variable ratio is of primaryimportance.

With the foregoing in mind, the present invention has for one object theprovision of control apparatus which utilizes the critical flow or sonicvelocity of a fluid through the throat of a properly shaped restriction,for example, such as a nozzle, to provide pressures which areproportionate to a pressure having large variations, but which arewithin such ranges as to permit utilization of movable wall means forthe actuation of control devices.

A further object is to provide control mechanism, wherein anorifice-nozzle or nozzle-nozzle combination, operating on the principlesof critical flow is utilized as a pressure divider, and is arranged toactuate a control device in response to changes in a characteristic of asensed medium, such as pressure, temperature, and the like.

Another object of the invention is to provide in devices of thecharacter described, a pressure divider having an improved nozzleconstruction containing a sharp leading or entrance edge.

Still another object of the invention is to provide improved pressureratio control apparatus, which is extremely reliable in operation,inherently resistant to temperature changes and vibration, moreeconomical to build, and which occupies less space, is simple ofconstruction but capable of rugged use.

To this end, the present invention in one form briefly comprises someform of mechanism arranged to operate in response to variations in apressure ratio, the ratio pressures being obtained by means of apressure divider utilizing an orifice-orifice and/or orifice-nozzlearrangement connected for series fluid flow through restricted passageswherein sonic velocity is obtained in the throats of the orifices andnozzles. The ratios of pressures are balanced by the ratios of the areasof movable wall means upon which the pressures obtained from thepressure divider are acting. Any change in the predetermined or functionvariable ratios is arranged to create an unbalance which is utilized toactivate some form of control device, or the atent ice pressure ratiomay be modified by a function of some other auxiliary control.

Further objects of the invention will be brought out in the followingpart of the specification, wherein detailed description is for thepurpose of fully disclosing several embodiments of the invention withoutplacing limitations thereon.

Referring to the accompanying drawings, which are for illustrativepurposes only:

FIG. 1 is a view schematically illustrating one form of controlapparatus embodying the features of the invention, arranged to operatein response to the changes in the ratio of inlet and outlet pressures ofa compressor;

FIG. 2 is a modified arrangement, wherein the compressor outlet pressureis modified in accordance with the differences in static and dynamicpressures;

FIG. 3 illustrates a further modification wherein the intermediatepressure is utilized as a multiplier in combinaton with an evacuatedbellows;

FEG. 4 is a view illustrating the present invention as applied to aspeed governor for a gas turbine; and

FIG. 5 illustrates another modified form, wherein the pressure ratio isvaried in response to temperature changes of a sensed medium.

Referring more specifically to the drawings, for illustrative purposes,FIG. 1 illustrates control apparatus according to the present inventionwhich embodies three component devices, namely, a pressure ratio sensingunit A, a pneumatic relay B and a valve actuator mechanism C.

As shown, the pressure ratio sensing unit A comprises a housingstructure ll) of suitable material, this housing having an internalvalve structure so formed as to provide internal cavities or chambers 11and 12 which in this instance form cylinders within which interconnectedpistons 13 and 14 respectively are supported for reciprocal movement.

The housing 10 is provided with a connection 15 which is connectiblewith a source of fluid pressure, shown in this instance as being acompressor outlet, and a connection 16 to atmosphere or the compressorinlet. The high pressure within the housing 15 is indicated by thedesignation P whereas the connection to atmosphere pressure is indicatedby P These pressures are interconnected within the housing throughparallel flow passages which respectively include the chambers 11 and12. The chamber 11 is intermediate an upstream nozzle 17 and adownstream nozzle 18, whereas the chamber 12 is intermediate an upstreamnozzle 19 and a downstream nozzle 25. The nozzles are so designed thatthe ratio of exit pressure to the entrance or upstream pressure is lessthan a critical value, depending upon the particular fluid beinghandled, and such that sonic velocity is attained at the throat. Byutilizing a divergent conical passage from the throat of the nozzle, aconsiderable recovery of pressure is obtained without altering thecritical ratio existing between the pressure at the throat of the nozzleand the entrance. By this means sonic flow will be obtained at thethroat of both nozzles although the ratio of exhaust or recoveredpressure at the end of the divergent nozzle to the total pressure at thenozzle inlet is higher than the critical pressure ratio required forsonic flow through the nozzle. This greatly extends the range ofoperation of the ratio sensing unit.

The sets of nozzles thus provided in each of the parallel flow pathsforms in effect a pressure divider which acts to form an intermediatepressure P in chamber 11 and an intermediate pressure P in chamber 12,which proportionally reflect over a smaller range variation in theupstream pressure P While the nozzles of the pressure divider may be asdescribed above, the linear operation of the divider may be extended atlower inlet densities of the fluid by providing a sharp inlet or leadingedge 21', as indicated in FIG. 1.

The intermediate pressures P and P may be adjustably controlled byadjusting the flow through either the upstream nozzle or the downstreamnozzle. In the present instance, provision is made for adjusting flowthrough the nozzle 18 by means of an adjustable needle valve 21, and theflow through nozzle by a needle valve 22.

In the arrangement shown, the intermediate pressures P and Prespectively act in the same direction against the interconnectedpistons 13 and 14- respectively, while the upstream pressure P andpressure P respectively act on the pistons 13 and 14 in the oppositedirection. Thus an arrangement is provided wherein a balanced conditionmay be established for a desired overall pressure ratio by adjusting theneedle valves 21 and 22.

First the needle valve 22 is adjusted so that the ratio of P to P is thesame as the ratio of the effective area of piston 13 to piston 14. Thus,the forces acting on the ends of the pistons 13 and '14 will be balancedfor all values of P sufficiently high to cause sonic flow in the throatof nozzles 19 and 20. Next, the needle valve 21 may be adjusted to givea pressure P so that the ratio of P to P times the ratio of theeffective areas of piston 14 to piston 13 is equal to the desiredoverall ratio of P to P A light spring 23 is provided so that duringmovement of the interconnected pistons 13 and 14, or other suitable wallmeans, such as interconnected diaphragms or the like, this spring willact to diminish instability or droop. Advantage may be taken of suchmovement of the piston structure to operate a suitable device. In thiscase, the movable wall means are connected to a valve 2'4 which isoperatively associated with a port opening 25 and controls the flow ofhigh pressure P fluid into a connection conduit 26.

The pneumatic relay B, as disclosed, comprises briefly an elongatehousing 27 Within which is reciprocally mounted a pair of valve members28 and 29 by which the high pressure P in a connection conduit 30 isselectively connectible with conduits 31 and 32, the conduit which isnot connected to the high pressure being vented through a port opening33.

One end of the housing 27 is arranged to form a pneumatic cylinder 34having operative relationship With a piston 35 which is connected to thevalve members 28 and 29. The left end of the cylinder 3 is connectedwith conduit 26, and is also vented to atmosphere through a restrictedpassage 36, and houses an expansion spring 37 which exerts a biasingaction upon the piston 35 tending to move the valves to a positionwherein conduit 31 is connected to the pressure P, and conduit 32 isvented to atmosphere.

The valve actuator mechanism C may assume varied forms of construction.In the present case, it is shown as comprising a hollow housing 38, aportion of this housing being arranged to form a pneumatic cylinder 39within which there is reciprocally mounted a piston 4% which is normallybiased toward the right end of the cylinder by an expansion spring 41.

Movements of the piston 40 are utilized to actuate a valve 42 positionedin a flow duct 43. The valve is swingably supported and carries anactuating crank arm 44 having its outer end connected with a togglejoint 45 which is interconnected with the piston 40 through a piston rod46. The conduits 31 and 32, it will be observed, communicate withopposite sides of the piston 40 and the valve 42 is arranged to be inclosed position, when the piston 46 is at the extreme position of itsmovement to the right end ofthe cylinder 39.

Having explained this embodiment of the invention in detail, theoperation will now briefly be considered. As before mentioned, theneedles 21 and 22 are adjustable so as to select any desired pressureratio above a certain minimum, such minimum being established by designcharacteristics of the particular nozzles. For example, if it is desiredto actuate the valve 42 in the opening direction when the pressure ratioreaches or exceeds 4:1, suitable values for the areas of pistons 13 and14 are so chosen that the piston 14 will be twice that of piston 13, andthe pressure ratio will be set to balance at the same ratio. Thepressure ratio selector needle 21 will then be set so as to cause toequal 2, in which case the overall ratio will equal 4:1. This wouldconstitute the balanced condition of the movable wall structure. Whenthe high pressure P, is below a value at which the balanced conditionobtains, the unbalance of the movable Wall structure is in a directionto maintain the valve 24 in the open position.

With the valve 24 open, the pressure P flows through conduit 26 to thecylinder 34', where, with the aid of the spring 37, it forces the piston35 and the connected valve members 28 and 29 to the right so that thepressure P is admitted through conduit 31 to the left side of the piston4d of the valve actuator mechanism C. The pressure P augmented by theforce of the spring 41 actuates the toggle joint 45 in a direction tohold the valve 42 in the closed position.

When the set ratio is attained, a balanced condition is reached in theratio sensing unit A, as explained hereinabove. If the high pressure Pincreases to a value above that at which the balanced condition obtains,the valve 24 is moved to a closed position to shut off flow throughconduit 26. Pressure in the cylinder 34 will then bleed through therestricted passage 36 to atmosphere permitting the pressure P; which isalways present at the other end of the housing 27 to move the valves 28and 29 to a position wherein valve 2% will connect the pressure P withconduit 32' to overcome the force of the spring 4 1 and move the piston41 in a lcftwardly direction to open the valve 42. It will beappreciated that the utilization of the pressure ratio sensing unit A inconnection with the other components just described is exemplary of onlyone manner of use of the sensing unit, and varied arrangements arepossible to meet the particular installation conditions.

Referring to FIG. 2, a modified construction of the sensing unit A isdisclosed which embodies the basic principles of the unit shown in FIG.1, except that diaphragms have been utilized instead of movable pistons.

The needle 21 has been deleted, and in its stead, there is provided aneedle 47 for controlling the flow through nozzle 17. This needle isoperatively connected with a diaphragm 48 or other equivalent structurehaving its opposite sides connected through connection conduits 49 and50 with a fluid flow duct 51 in such manner that the needle will modifythe flow through the nozzle 17 and consequently the intermediatepressure P in accordance with dynamic and static conditions of the fluidin the duct 51. The diaphragm 4 8 is also acted upon by associated coilsprings 52 and 53 respectively positioned on opposite sides thereof.

When the modified form of construction described above is supplied witha flow of fluid through the duct 51 it will operate the valve 24 tomaintain the desired ratio between the total pressure P which is sensedby the conduit 49 and the pressure P which is the inlet pressure of thecompressor. The sensing unit operates in the same manner as describedabove for FIGURE 1, the only difference being that the needle 47 whichcontrols the area of the orifice i7 is positioned in response to theflow through the duct 51. When the how through the duct 51 is materiallyreduced so that it approaches the point of no flow it will indicate thetendency of the compressor to surge or hunt. Under these conditions oflow flow the difference between the total pressure sensed by the conduit59 and the static pressure sensed by the conduit 50 will besubstantially reduced thus allowing the spring 53 to move the needle 47in a direction to reduce the area of the nozzle 17. The reduced area ofthe nozzle 17 will increase the ratio between the inlet pressure P andthe pressure P This increased pressure ratio will decrease the pressureP and permit the spring 23 acting on one side of the smaller diaphragmto open the valve 24, which movement can be used to position a by-passvalve to control the surging condition of the compressor.

Referring now to FIG. 3, this embodiment of the invention is similar tothe arrangement shown in FIG. 2, insofar as it provides for modificationof the intermediate pressure P in accordance with dynamic and staticconditions of the high pressure fluid. This arrangement, however,differs in that instead of utilizing nozzles arranged to provideparallel connected pressure dividers, only two nozzles in this case areused to provide a single pressure divider. Further, in this embodiment,a single diaphragm 61 is utilized, and one side of the diaphragm 61' isconnected to an evacuated bellows 54' having its exterior subject to theintermediate pressure P while the other side of the diaphragm is subjectto the lower pressure P thus introducing an absolute pressure factorinto the differential pressure ratio acting on the diaphragm, sucharrangements being conventional in pressure ratio control devices.

Referring to FIG. 4, there is disclosed an arrangement wherein thefeatures of the present invention are arranged as a governor inconnection with gas turbine installation in which a turbine 55 isconnected with a compressor 56, the compressor being utilized to furnishcombustion air to a combustion chamber 57 wherein fuel is delivered to asuitable nozzle 58 through a supply conduit 59 from fuel pump 69.

In this case a sensing unit A is arranged with a single set of nozzlesto provide a pressure divider similar to the arrangement shown in FIG.3. Also, it will be perceived that a single diaphragm 61 is connectedwith an evacuated bellows 54 and subject to the intermediate pressure onone side of the diaphragm, the other side of the diaphragm likewisebeing subject to atmospheric pressure.

In this form of the invention, however, provision is made for modulatingthe intermediate pressure in response to temperature changes. For suchpurposes, a needle 62 is associated with the nozzle 18. This needle isshown as being biased in one direction by an associated spring 63, andin an opposite direction by bellows 64 which is operatively connectedthrough a capillary tube connection 65 with a suitable temperaturesensor 66 mounted in the intake of compressor 56.

Movements of the diaphragm 61 are utilized to control a valve 67, whichis normally held in closed position by a spring 68, this valve in openposition acting to connect a chamber 69 on one side of diaphragm 79 withatmosphere. The chamber 69 is connected through an orifice 7ll with thepressure upstream of nozzle 17,

6 which pressure in this case is the outlet pressure of compressor 56obtained through conduit connection 72.

The other side of the diaphragm 70 is exposed to chamber 73 which is incommunication with a bypass around the fuel pump 69 composed of a valveseat opening 74, chamber 75 of unloading valve 76 and conduit 77.

The movements of diaphragm 70 are utilized for actuating a fuel servometering valve 78 which is normally biased by a spring 79 intoengagement with the diaphragm 79, and to open position completing apassage through conduits 8t) and 81 from the chamber 75 to the interiorof a bellows 82 which carries valve 83 at its free end, this valve beingoperatively associated with the valve seat opening 74 and biased towardseating position by means of a coil spring 84. The valve 83 has arestricted discharge orifice 85 therein.

Provision is also made for bleeding chamber 69 through a conduitconnection 86 having a valve 87 arranged to open and close in responseto temperature variations in the discharge from the turbine 55.

In operation, the needle 62 which controls the area of the orifice 18 ismodulated in response to temperature changes of the fluid at the sensor66. Thus, if the temperature of the fluid at the sensor 66 falls below apreselected temperature, the needle 62 is moved away from the orifice 18to permit greater flow through the orifice and thus decrease theintermediate pressure between the orifices 17 and 18. As theintermediate pressure decreases the evacuated bellows 54 will expand tomove the valve 67 toward open position, to the right as shown in thedrawing, permitting fluid to flow from the chamber 69 to atmosphere. Asthe pressure in chamber 69 decreases the diaphragm 70 will move to theleft, as shown in the drawing, and the servo metering valve 78 will bepositioned to throttle the flow of fuel through the conduit 81 tomaintain an optimum operating condition.

As the temperature of the fluid at the sensor 66 returns to thepreselected temperature, the increase in the temperature of the fluidwill cause the sensor means to move the needle 62 toward the orifice 13and the intermediate pressure between the orifices 17 and 18 willincrease. As the intermediate pressure increases, the evacuated bellows54 will contract and the valve 67 will be moved toward the closedposition, to the left as shown in the drawing, thus increasing thepressure in the chamber 69. The resulting increase in pressure inchamber 69 will move the diaphragm 70 to the right and the servometering valve 78 will be repositioned to restore the system to thedesired operating condition.

FIG. 5 illustrates still another modfication in which a temperaturesensing element is utilized. The arrangement in this case similarlyemploys two pressure dividers in parallel flow passages in the samemanner as shown generally in FIG. 1. However, in the arrangement of FIG.5, the nozzle 2-0 has been replaced by an orifice 88 and the needle 22is associated with the nozzle 19 for utilization in regulating theintermediate pressure P Flow through the orifice 88 is modulated inresponse to temperature changes of a sensed medium by means of a valvedisk 89 arranged for limited movement within a sleeve member 96supported for guiding movement within a chamber 91. The valve disk 89 isbiased by a spring 92 toward a limiting position of movement, and thesleeve is likewise biased in a downward direction by a spring 93 with aportion of the sleeve making pressure engagement with one end of a rodmember 94 which extends into a tubular extension 95 of the housing andhas its other end bearing against the associated end of the extension.Also provided in the housing 95 is a vent opening to permit the fluidflowing from the orifice 88 to escape to the surrounding atmosphere. Therod 94 and the housing 95 are constructed of materials having differenttemperature coefficients, the housing extension being of a materialwhich will expand a greater amount '2? than the material of the rod 94for a specific temperature change. Thus, with temperature increases, theeffect is to move the valve disk 39 away from the open end or" orifice88 to permit greater flow and thus decrease the intermediate pressure PThe two pressure dividers are connected to form a pneumatic bridge inwhich the nozzles 17 and 1'9 are connected to a common source ofpressure, and in which the intermediate chambers between the nozzles ofthe pressure dividers are respectively connected through conduits 96 and97 with chambers 109 and ill-ti of a balanced pilot valve in which theintermediate pressures P and P of the pneumatic bridge are applied onthe opposite sides of a diaphragm 2% carried by a shaft 111. The shaft1H also carries a diaphragm 1112 positioned between and separatingchambers 113 and 114, these chambers being interconnected through apassage containing a restricted orifice 15. The shaft is supported forreciprocable movements on small spaced flexible supports which consistof an intermediate diaphragm 116 between the chambers 11% and 133, adiaphragm 117 adjacent one end of the shaft and separating chamber 114from ambient atmosphere, and a diaphragm 118 adjacent the other end ofthe shaft and separating chamber Hi9 from ambient atmosphere. The normalaxial position of the shaft 111 is determined by means of suitablebalancing springs 119 mounted at the shaft ends, one of which may beadjusted by means of a threaded stud structure 120.

A bleed valve 99 is arranged to control how from a conduit 1% toatmosphere in response to movements of shaft ill. The valve 99 has astem 121 which abuts the diaphragm 12%, the Valve being biased towardsseating position by an expansion coil spring 122.

The conduit tee is in communication with the chamber 1M through arestricted orifice 123 and also with one side of a diaphragm 161 of anactuator 102., which has a stem 1G3 operatively connected with a valve364 for controlling in this case the flow ot a fluid, for example,through a pipe 195. The opposite side of the diaphragm 191 is engaged byan expansion spring res which biases the diaphragm in a direction tocause opening of the valve upon decrease or" the pressure acting on theopposite side of the diaphragm from that upon which the spring 166bears. As shown, pressure is supplied from the high pressure sourcethrough a conduit 197, a restricted orifice 103, and thenc throughconduit ice for motivating the actuator 62 in response to the operationof the valve 99.

Thus, in the arrangement shown, when the diaphragm 9?; is in a balancedcondition and the disk valve 35 is moved toward the orifice 8%, thepressure P is increased and acts to force the diaphragm 98 upwardly toopen the valve 99 so as to reduce the pressure acting on the diaphragm 1M. Upon reduction of this pressure, the spring MP6 forces the diaphragmdownwardly and causes the valve 194 to move toward a position ofincreased opening. Conversely, upon increased temperatures of the sensedmedium, the valve Mi -i will be actuated toward closed position. It ispossible that a condition may arise wherein the pressure in conduit 107may suddenly vary without there being a change in differential pressureacting on the diaphragm 8, which would tend to open or close the valve99. As a consequence, this change in pressure would tend to be carriedthrough the conduit 1% and cause the actuator 192 to eifect an undesiredchange in the position of valve 164. This is counteracted by astabilizing action of the pilot valve. For example, if the pressureincreases in conduit 1%, tending to close valve ltl l, there will be aflow through the restricted orifice 123 into chamber 114, wherein theincreased pressure will act through the diaphragm 112 and move shaft 111in a direction to slightly increase the opening of valve 99 and thusrestore the pressure in conduit 1% and reestablish the position of thevalve 164.

From the foregoing, it will appreciated that the 0 an present inventionis susceptible of varied embodiments depending upon the particularinstu. in which it is to be utilized, and that as a consequence variousother modifications may suggest themselves to those skilled in the artwithout departing from the spirit of the invention disclosed herein. Itis therefore not desired to restrict the invention to the specific formor forms shown or uses mentioned, except to the extent indicated in theappended claims.

I claim:

1. Control apparatus, comprising: a pair of parallel flow passagesconnected between a high pressure fluid source and a low pressure fluidsource; a pair of spaced sonic velocity flow restrictions in each ofsaid flow passages, whereby intermediate flow pressures are efliected;movable wall means including areas having said intermediate pressuresacting thereon in one direction, and said high and low pressures in anopposite direction; a control device varied in response to movement ofsaid wall means; and means for adjustably regulating flow through atleast one of said flow restrictions of each pair.

2. Control apparatus, comprising: sonic flow pressure divider meansconnected between a high fluid pressure and a low fluid pressure toprovide at least one intermediate fiuid pressure; movable wall meansbalanced by a predetermined value of said intermediate pressure actingon one side and predetermined values of said high and low pressuresacting on the opposite side thereof; means biasing said wall means inone direction, during said balanced condition; a fluid operatedactuator; and valve means responsive to movements of said wall means forcontrolling said actuator.

3. Control apparatus, comprising: a pair of parallel flow passagesconnected between a high pressure fluid source and a low pressure fluidsource; a pair of spaced sonic velocity flow restrictions in each ofsaid flow passages, whereby intermediate flow pressures are effected;novable wail means including areas having said intermediate pressuresacting thereon in one direction, and said high and low pressures in anopposite direction; a control device varied in response to movements ofsaid wall means; and means for modifying one of said intermediatepressures in accordance with variations in static and dynamicdifferences of said high pressure fluid.

4. Control apparatus, comprising: sonic flow pressure divider meansconnected between a high fluid pressure and a low fluid pressure toprovide an intermediate liuid pressure; movable wall means acted upon bysaid pressures and arranged to be balanced at predetermined values ofsaid pressures; means biasing said wall means in one direction, duringsaid balanced condition; a control device varied in response tomovements of said wall means; and means for modifying the action of saidpressures on said movable wall means in accordance with variations instatic and dynamic characteristics of said high fluid pressure.

5. Control apparatus, comprising: a pair of parallel flow passagesconnected between a high pressure fluid source and a low pressure fluidsource; a pair of spaced Sonic velocity flow restrictions in each ofsaid flow passages, whereby intermediate flow pressures are effected;movable wall means including areas having said intermediate pressuresacting thereon in one direction, and said high and low pressures in anopposite direction; and a control device varied in response to movementof said wall means.

6. Control apparatus, comprising: sonic flow pressure divider meansconnected between a high fluid pressure and a low fluid pressure toprovide at least one intermediate fluid pressure; movable wall meansbalanced by a predetermined value of said intermediate pressure actingon one side and predetermined values of said high and low pressures onthe opposite side thereof;

means biasing said =wall means in one direction, during said balancedcondition; and a control device varied in response to movements of saidwall means.

7. Control apparatus, comprising: a pair of flow passages connectedbetween a high pressure fluid source and a low pressure fluid source; apair of spaced restricted sonic velocity flow openings in each of saidflow passages, whereby an intermediate pressure is effected between eachof said pair of flow openings; movable wall means including areas havingsaid intermediate pressures acting thereon in one direction and saidhigh and low pressures acting in an opposite direction; and means forvarying one of said restricted openings in response to a characteristicof a sensed medium.

8. Control apparatus, comprising: a pair of flow passages connectedbetween a high pressure fluid source and a low pressure fluid source; apair of spaced restricted sonic velocity flow openings in each of saidflow passages, whereby an intermediate pressure is efiected between eachof said pair of flow openings; movable wall means including areas havingsaid intermediate pressures acting thereon in one direction and saidhigh and low pressures acting in an opposite direction; and means foradjusting at least one of said restricted openings to pre-set theintermediate pressure in said passage to a desired value.

9. Control apparatus, comprising: a pair of flow passages connectedbetween a high pressure fluid source and a low pressure fluid source; apair of spaced restricted sonic velocity flow openings in each of saidflow passages, whereby an intermediate pressure is efiected between eachof said pair of flow openings; movable wall means including areas havingsaid intermediate pressures acting thereon in one direction and saidhigh and low pressures acting in an opposite direction; and means foradjusting one of the restricted openings in each of said flow passagesto pre-set said intermediate pressures for preselected values.

10. Control apparatus, comprising: a pair of parallel flow passagesconnected between a high pressure fluid source and a low pressure fluidsource; a pair of spaced sonic velocity flow restrictions in each ofsaid flow passages, whereby intermediate flow pressures are efiected;movable wall means including areas having said intermediate pressuresacting thereon in one direction, and said high and low pressures actingin an opposite direction; and a control device varied in response tomovement of said wall means.

11. Control apparatus for maintaining the pressure ratio between firstand second pressures at a selected value, comprising: a pair of flowpassages positioned between said first and second pressures; a pair ofspaced restricted sonic velocity flow openings in each of said flowpassages, whereby an intermediate pressure is effected between each ofsaid pair of flow openings; movable wall means including areas havingsaid intermediate pressures acting thereon in one direction and saidfirst and second pressures acting in an opposite direction; means foradjusting one of the restricted openings in each of said flow passagesto pre-set said intermediate pressures; and control means actuated inresponse to movement of said Wall means to maintain the pressure ratiobetween said first and second pressures at the selected value.

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